Reservoir Fluid Sampling In Unconsolidated Sand To Assess Recovery Uncertainty

Texaco North Sea UK Co.'s Captain field comprises unconsolidated sands, containing high viscosity oil, 50-150 cp. Four delineation wells were drilled prior to the second phase of horizontal development drilling. A reservoir fluid sampling campaign was designed to remove uncertainty in fluid quality, in particular, variations in viscosity, bubble point and GOR under reservoir conditions. Viscosity is a critical parameter for recovery estimation in a viscous oilfield dependent on water injection for oil recovery. The four-well program also had gas and water sampling objectives. Fluid sampling was done with the Modular Formation Dynamics Tester (MDT)t using the dual packer module, in two wells. As a result, the drawdown was reduced to below 40 psi and mobilization of the sand particles during cleanup and sampling was avoided. The packers remained set for 9 and 13 hours at two separate zones with no tool sticking. More than 50 samples were recovered total from the 4 wells, using the low shock sampling technique. Single-phase bottles were used to have only liquid phase at the surface and laboratory conditions. The packer tests also provided transient pressure buildup data, which were analyzed for formation permeability. Analysis of the samples provided information on reservoir compartmentalization and a good handle of viscosity values that were used for recovery estimations under water injection. Confidence in the dual packer buildup results resulted in canceling coring operations in the upcoming wells, replacing core with MDT dual packer permeability tests.